The present invention is generally directed to the manufacture of medical devices. More specifically, the present invention includes a system and method for manufacturing bilayer adhesive patches that are to be bonded to a medical device that is formed in such a manner that patching is required to complete the manufacture of the device.
Current manufacturing processes to make many medical implantable devices involve forming thin silicone elastomer shells by dipping or molding a thin layer of silicone material on a male mandrel. For example, in the manufacture of breast implants, the outer silicone membrane is formed on a male mandrel. The membrane, typically called a “shell,” is removed from the mandrel by cutting a small hole in the shell so that the shell can be removed from the male mandrel without deforming or tearing the shell. Through the hole, the surrounding edges of the shell can then be grasped to stretch and peel the remainder of the shell from the male mandrel more easily. After the shell is off of the mandrel the small circle or hole must be patched to close the shell so as to provide full containment integrity to the shell so that it may then be filled with a filling material, such as a silicone gel.
Current processes of making bilayer patches for medical and cosmetic implants and prosthetics are more difficult, costly, and time-consuming than they need to be. Patches for devices such as breast implants formed from silicone usually have a first layer that is vulcanized, which is then applied to a second layer that is unvulcanized.
Vulcanization generally refers to the process of crosslinking the silicone polymer based material to form a dry, non-adhering material with good elastomeric memory. The vulcanized layer is thin, typically less than 0.5 mm and preferably less than 0.2 mm. Forming thin layers with sticky unvulcanized silicone elastomer bases is difficult and typically done by calendering or solvent based knife-coating, with subsequent devolatilization and vulcanization on a sheet of base plastic such as Teflon® (sold by DuPont), polyester or polyethylene.
The unvulcanized portion or layer of the bilayer patch, typically less than 0.5 mm thick, is typically applied to the vulcanized layer by calendering unvulcanized silicone into a thin layer and then applying that layer to the vulcanized layer described above. Calendering refers to the process of forming a uniform thickness thin layer by pressing uncured malleable elastomer systems between rotating cylinders or rollers. It is difficult to peel thin layers off of the rollers used for calendering without tearing or breaking the fragile thin unvulcanized layer. Accordingly, this process often results in a high loss factor. Alternatively, the unvulcanized layer can be applied to the vulcanized layer by a solvent dispersion technique and subsequently devolatilizing the assembly before proceeding with applying the patch to the shell to close the opening cut into the shell to remove shell from the mandrel. After the vulcanized and unvulcanized layers are joined, they are typically supported on a thin plastic sheet.
Regardless of how the vulcanized silicone layer and unvulcanized silicone layer used to form the patch are combined, once combined both sides are typically covered with a thin layer of a thermoplastic polymer such as polyethylene. The polyethylene covered bilayer sandwich is then cut into the desired size and shape for the patch.
Consistent with current modern manufacturing procedures, the patches are then transferred to another work area in which an assembler manually peels off the polyethylene coating and applies the patch to the shell by placing in into the shell, vulcanized side away from the hole and unvulcanized side facing the hole. Vulcanization and bonding are typically achieved by applying heat and pressure to the assembly.
Another technique that has been investigated for the manufacture of thin patches is the use of injection molding to form the patch. Injection molding of silicone elastomers and plastics is common practice and a well-developed art, though it may also be used for other materials. A wide variety of products are manufactured using injection molding, which vary greatly in their size, shape, complexity, and application.
“Green strength,” a measure of tack, deformability, elastic memory and malleability of the unvulcanized silicone elastomer base is a relevant limiting factor to injection molding. Moderate green strength silicone materials typically used in forming silicone elastomer shells do not easily lend themselves to typical mixing systems such as two roll milling (calendering) or pumpable paste static mixer systems.
Green strength can be a good indication of processing behavior and a moderate to high green strength is desirable in processing operations in which it is important to maintain the integrity of a shape piece of material, particularly for the unvulcanized layer.
Thick preforms of high green strength unvulcanized silicone, typically formed by continuous extrusion and chopping, are commonly used in industrial processes. However, injection molding of thin preforms having moderate green strength and tack is not known to have been done before commercially for this application on account of the adhesion between a thin preform of unvulcanized silicone and common mold materials (e.g. aluminum or steel) being too strong to provide a reliable release that preserves the integrity of the thin preform upon removal from the mold. Injection molding of thin preforms is not commercially practical when losses due to the preforms being damaged, deformed, or partially stuck to the mold are too costly.
There is a need for an improved method for forming thin bilayer silicone patches that is less expensive, less labor and time intensive, and that reduces the loss factor of material waste. For example, the traditional process of removing the polyethylene coating is tedious and transporting the patches from one work station to the next for processing creates delays, inefficiencies, and increased costs for labor and facilities. It would be desirable to provide an improved method for forming implant and prosthetic patches in which the patch assembler is able to mold the patches on demand at a single work station. It would be especially desirable to provide a method for injection molding of thin preforms that preserves the integrity of the preforms upon removal from the mold. The present invention satisfies these and other needs.